As suggested, lots of wire. Doesn't matter to the RF if
it's insulated or not (it will matter as far as corrosion
is concerned). No soldering, use heavy clamps instead. Coat
with some kind of noncorrosive 'goop'. Bury it deep enough
so the lawn mower doesn't get into it. when you think you
have enough metal in the ground, add another mile...
'Doc

PS - If you're going to use 136 Khz, make that 10 miles.